Rotational molding, also known as rotomolding, is a well known process which is widely used to produce hollow plastic parts such as gasoline containers, garbage cans, agricultural storage vessels, septic tanks and sporting goods such as kayaks. The process is undertaken by loading a charge of finely divided plastic resin into the mold “shell”, then rotating the mold (usually, on two axes) while heating it to a temperature above the melting point of the plastic resin. The melted plastic flows through the mold cavity under the forces caused by the rotation of the apparatus. The rotation continues for sufficient time to allow the molten plastic to cover the surface of the mold. The mold is then cooled to permit the plastic to freeze into a solid. The final stage of the molding cycle is the removal of the part from the rotomolding machine.
The time required to complete the molding cycle is a function of the bulk properties of the plastic which is being molded. For example, it is recognized by those skilled in the art that the plastic resin which is charged into the mold is preferably finely divided (i.e. ground into powder) and has a high bulk density and a narrow particle size distribution to facilitate the “free flow” of the resin.
It will also be appreciated that the physical properties of the rotomolded part are influenced by the use of a proper molding cycle time with “undercooked” parts having poor strength properties and “overcooked” parts suffering from poor appearance (a “burnt” color) and/or a deterioration of strength properties. It is desirable to have a short molding cycle (so as to improve the productivity of the expensive rotomolding machinery) and a broad “processing window” (i.e. the rotomolding composition ideally provides “properly cooked” parts in a short period of time but does not become “overcooked” for an extended period of time).
In addition, the properties of the rotomolded part are affected by the molecular structure of the polymer used to prepare the part. Physical properties of importance include stiffness (as indicated by the modulus of the part), environmental stress crack resistance (or “ESCR”), impact resistance and resistance to warpage.
Thermoplastic polyethylene copolymer is a commonly used plastic resin for the manufacture of rotomolded parts. Conventional polyethylene copolymer resin (which is generally prepared by the copolymerization of ethylene with a C4 to 10 alpha olefin in the presence of a chromium catalyst or a Ziegler Natta catalyst) is typically used. The alpha olefin comonomer produces “short chain branches” (SCB) in the copolymer. These SCB reduce the crystallinity of the copolymer (in comparison to a linear ethylene homopolymer) and the copolymers typically have improved impact resistance in comparison to homopolymers. These conventional polyethylenes may be referred to as “heterogeneous” in the sense that he polyethylene is actually a mixture of different polyethylene chains having significantly different molecular weights and comonomer distributions. Most notably, a conventional heterogeneous polyethylene copolymer resin generally contains three fractions:
i) a low molecular weight fraction having a high comonomer content (or high level of SCB)—this fraction is often referred to as “wax” or “extractables”;
ii) a very high molecular weight fraction having little or no comonomer—this fraction is often referred to as “homopolymer”; and
iii) a fraction of intermediate molecular weight and SCB content.
These conventional polyethylenes are well suited for rotomolding. For example, whilst not wishing to be bound by theory, it has been postulated that the high molecular weight “homopolymer” fraction may enhance the stiffness or modulus of the rotomolded parts. However, as may be expected, the physical properties of a finished part (which has been molded with a proper cycle time) are largely “set” by the molecular structure of the polyethylene copolymer used to prepare the part. This, in turn, often results in finished parts with a suboptimal balance of properties. Persons skilled in the art of rotational molding have addressed this problem by preparing blends of ethylene copolymers to produced parts have a more desirable balance of properties. For example, U.S. Pat. No. 5,082,902 (Gurevitch et al.) discloses a rotomolding composition prepared from 50 to 95% of a higher density ethylene copolymer with 5 to 50% of a lower density ethylene copolymer. The copolymers disclosed by Gurevitch et al. are conventional (heterogeneous) resins, and the blend components have significantly different densities.
The rotomolded parts described in the Gurevitch et al. patent are said to have better impact properties, improved warpage resistance and better ESCR in comparison to parts prepared from a single resin.
Similarly, U.S. Pat. No. 5,530,035 (Needham) teaches a rotomolding composition made from a blend of ethylene polymers having significant different densities.
More recently, homogeneous ethylene copolymers have become commercially available. These homogenous polymers have a uniform (or narrow) molecular weight distribution and a uniform comonomer distribution. This, in turn, causes the homogeneous copolymers to have a well defined melting point (in comparison to the heterogeneous polymers which have a melting point “range”, or even multiple melting points).
The sharp melting point might have been expected to be advantageous for a rotomolding process. However, in practice, it has been found that parts prepared from a single homogeneous resin are prone to warpage and also generally have poor modulus or stiffness. Blends of heterogeneous ethylene copolymers with homogeneous ethylene polymers have been proposed as a means to resolve these problems and it is believed that some such blends are in commercial use. U.S. Pat. No. 5,858,491 (Gussens et al.) teaches the preparation of a hollow molded article from a polymer blend comprising homogenous ethylene copolymer having a low density and a heterogeneous ethylene polymer having a very high density.
Stehling et al. teach that blends of homogeneous ethylene copolymers having substantially different molecular weight (such that the molecular weight distribution of the blend is greater than 3) provide films with good tear strength. Stehling et al. do not teach that blends having a narrow molecular weight distribution are suitable for rotomolding.
We have now discovered that blends comprising at least two homogeneous ethylene copolymers having very similar molecular weight and density provide excellent rotomolding compositions. More particularly, we have discovered that a very large improvement is obtained in the physical properties of the finished parts is obtained through the use of blend components having only small differences in comonomer distribution and/or molecular weight.